1. Field of the Invention
The invention relates to a device for producing tubed, bored piles, in which during sinking simultaneously a tubing and an inner auger are introduced into the ground in a rotary boring method.
2. Description of Related Art
Numerous production methods exist for producing bores for bored piles.
If the soil is soft, displacement bored piles can be produced. In the case of loose soils and when ground water is close to the surface, use is generally made of tubed boreholes. For this purpose a tubing is turned into the ground and the soil cropping up within the tubing is removed by differing boring tools. If there is no ground water, in order to avoid a hydraulic soil movement during soil removal from the interior of the tube, water must be filled into the latter. This method is time-consuming due to the need of constantly topping up the water.
If the production output is to be improved, drilling takes place with a continuous soil auger in the case of gravelly and sandy soils with ground water, so that the auger is at least as long as the borehole is deep. The auger is turned into the ground and the auger helixes and the soil located thereon support the wall of the borehole. This has a comparable effect to the production of a tubed bore. On reaching the final depth the auger is retracted essentially without rotating the same and simultaneously concrete is introduced under pressure into the cavity formed through the auger core tube.
In some soils, where the bored pile must grip in firm soil layers or it is necessary to cut through cohesive or harder soil layers, untubed production of bored piles with a continuous auger is less suitable, because as a result thereof, during the boring through or gripping in the hard soil, more material than is needed is delivered from the loose soil layers.
In some cases it is then possible to use boring methods in which simultaneously a continuous auger and an encasing tubing are introduced into the ground. Both the auger and the encasing tube must be at least as long as the depth of the borehole to be produced. DE 197 38 171 A1 describes a device suitable for this.
Such methods are known as double or twin head boring. There are two drive units, which on the one hand drive the inner, continuous auger and simultaneously the outer tubing. As a function of the method the auger and the tubing are rotated in the same direction or in opposite directions. It is also appropriate at least over a short area to axially displace the inner auger against the outer tubing.
The concreting procedure in the case of double head boring is similar to that with a continuous soil auger. On retracting the tubing, including the inner auger, as a rule concrete is pumped into the resulting cavity via the core tube.
However, it is not always possible without problems to feed soil during the sinking of the tubing through the inner, continuous auger. If in the case of loose soils layers of cohesive soil material are encountered, this can give rise to problems in the feeding or delivery procedure. The cohesive soil becomes stuck in the auger, forms a plug and the material flow within the tubing is no longer ensured. The auger rotates on the spot without delivering material upwards.
As a result of the auger blockage, in the worst possible case a bore must be broken off and the entire tubing with the auger has to be extracted for cleaning purposes. The subsequent boring of the pile can give rise to disadvantages with respect to the support behaviour of said pile, because the surrounding soil has been excessively loosened.
Another problem arises on boring in coarse-grain soils. In such a case the material to be fed can jam between the auger and the inner wall of the tubing and only with very great force expenditure can the auger be turned in the interior of the tubing and only a small amount of soil is delivered. Thus, the boring tool can only penetrate the soil very slowly.